The overall goal of Project 1 is to understand the nature of the immune response to AAV vectors in humans. This line of investigafion is based on a clinical trial of AAV-Factor IX (F.IX) administered to the hepatic artery in men with severe hemophilia B, in which we documented the simultaneous occurrence of loss of F.IX expression, and transient liver transaminase elevafion, beginning 3-4 weeks after vector injecfion. In the previous funding period we documented: 1) that the rise and fall in liver enzymes was accompanied by expansion and contraction of a populafion of capsid-specific CD8'' T cells, but no T cell response to F.IX; 2) that a substantial proportion of normal human subjects harbor AAV capsid-specific T cells as documented by IFN-7 ELIspot; 3) that human hepatocytes can process and present preformed capsid antigen on the surface ofthe transduced cell; 4) in preliminary studies, that the clinically approved proteasome inhibitor bortezomib reduces capsid antigen presentation on the surface ofthe transduced cell; 5) in non-human primates, and one human subject thus far, that the immunosuppressive regimen MMF/rapamycin can be safely coadministered with AAV vector delivered to the hepatic artery. In the next funding period, we propose three aims to build on these observations by; 1) monitoring and characterizing the immune response to the AAV capsid in human subjects undergoing AAV-mediated gene transfer in seven different AAV trials. We will use ELISpot for screening and polyfuncfional T cell analysis for more comprehensive assessment of T cell responses; we will also obtain a complete serum cytokine expression profile over fime in the subjects studied; 2) determining the levels and the biological significance of capsid antigen presentation of alternate AAV serotypes and capsid variants carrying mutations in surface-exposed tyrosine residues; these experiments will be carried out in a fully-humanized in vitro system; and 3) investigating new strategies to block or reduce antigen presentation through a pharmacologic treatment with bortezomib or exploiting the molecular mechanisms of viral immune evasion. Regulatory T cells will also be tested as modulators of capsid T cells. Aims 1 and 2 will involve extensive interacfion with Project 2, and Aim 3 with Project 3.